Continuous measurement of yarn twist

ABSTRACT

A method and apparatus for non-destructively measuring twist in a translating yarn (2) by means of a blade (1) which is in rubbing contact with the translating yarn (2) so that vibrations are imparted to the blade (1). The frequency of vibration of the blade (1) provides a measure of the twist imparted to the yarn (2) so that the signal frequency can be used as a measurement of the twist in a yarn (2) under test or as a feedback signal controlling the rate of twist imparted by a twist insertion station(8) during the formation of a yarn.

TECHNICAL FIELD

The present invention relates to a method and means for measuring theamount of twist in a travelling length of yarn and in particular to anon-destructive method for carrying out twist measurement.

BACKGROUND ART

Existing methods and means for measuring twist in yarn, for the mostpart, rely on the measurement of the twist angle alone and take noaccount of the important fact that the twist being measured is dependentupon the ever-varying yarn diameter. Such methods include a ply-twistmethod of measurement wherein a sample of yarn is untwisted in order todetermine the twist that has been imparted to the yarn. Another methodprovides an untwist-twist technique wherein the yarn extends while beinguntwisted and contracts while being twisted in the opposite sense of itsoriginal twist so that when the original length of the yarn is attainedduring twisting in the opposite sense the total number of revolutionsfrom untwist to twist is deemed to be twice the total twist of thelength of yarn under test. Another method as shown in G.B. No. 1,266,450proposes measurement of twist as the yarn is being formed at a pointupstream of the twist spindle while the yarn is rotating.

The existing devices and methods use one of the following techniques:

(1) obtain a measurement of twist from two measuredquantities--twist-angle and the yarn "diameter";

(2) obtain a measure of twist as the ratio of the specimen'stendency-to-rotate to the yarn speed necessitating the specimen to beactually in the process of twist insertion;

(3) use the twist-angle as a measure of the yarn twist ignoring thevariation in the yarn "diameter".

DISCLOSURE OF INVENTION

The present invention proposes a method and means of measurement oftwist in a rotating or non-rotating continuous travelling yarn which maybe effected subsequent to the formation of the twisted yarn.

In one aspect the present invention provides a method of measuring twistin a translating yarn comprising sensing the frequency of vibration of ablade having an edge in contact with the translating yarn andcalculating the twist of the yarn which is a function of the frequencyof vibration of said blade.

In another aspect the present invention provides apparatus for measuringtwist in a translating yarn comprising a blade adapted to have an edgein contact with a translating yarn, means for sensing the vibrationimparted to said blade by the translation of said yarn across said edge,and means for measuring the frequency of said vibration.

This invention measures the twist in a translating yarn, whether it isrotating or not, continuously and non-destructively, by sensing thefrequency of vibration of a blade which is in contact with the movingspecimen, and amplifying, processing and displaying or printing thetime-varying signal obtained therefrom.

The twist estimated by the new device and method is an accurate andprecise measure of twist of a running yarn as will be appreciated by thefacts:

(1) what is being sensed by the vibrating blade is the real twistitself;

(2) the measureand is independant of the yarn "diameter" variation; and

(3) only a single measurand is needed.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a schematic view of apparatus in accordance with a firstembodiment of the present invention;

FIG. 2 is a schematic view of apparatus in accordance with a secondembodiment of the present invention; and

FIG. 3 is a schematic view showing an example of how the apparatus ofFIG. 1 or FIG. 2 can be used during the production of twisted yarn.

MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 which shows a vibration sensing blade 1 with itsedge 1' placed so as to remain in contact with moving yarn 2. Blade 1vibrates as yarn 2 moves across edge 1'; such vibration depends uponblade design and dimensions, yarn tension, speed and surface propertiesbut mainly the surface twist angle α. By turning blade 1 around itspivot axis, a maximum amplitude of vibration will be evident where theblade edge 1' is parallel to yarn elements on the surface of yarn 2(e.g. filaments and fibres in case of a single yarn, or single yarns incase of ply yarns). Thus at the maximum amplitude position the anglebetween blade edge 1' and the axis of yarn 2 will indicate the yarn'ssurface twist angle α as represented by twist-angle pointer 4 againstscale 5.

For an ideal case where the yarn has constant twist and constantdiameter, i.e. the surface twist angle α has a constant value, the blade1 should have only one position at which blade vibration has its maximumamplitude giving the twist angle α. Furthermore, at constant yarn speedand tension the vibration frequency would be constant and correspond tothe constant amount of twist. In the ideal case the yarn twist could bereadily calculated using α and the yarn diameter according to thefollowing:

    T=(tan α)/πd)                                     (1)

T=yarn twist in turns/cm

d=yarn diameter in cm

As yarns in practice have variable diameters and twist, i.e. variablesurface twist angle α , then the procedure adopted with the apparatus ofFIG. 1 is to adjust blade 1 and fix it at a specified position. Yarntwist is measured by sensing the frequency of the blade vibration. Theselected blade position has its effect on the vibration amplitude. For arelatively long blade, as shown in FIG. 1, it is preferable to fix theblade with its edge as close as possible to the maximum amplitudeposition; i.e. the angle between the blade 1 and the axis of yarn 2 isas near as possible to the nominal twist angle α. For a very shortblade, as shown in FIG. 2, it may be mounted with its edge perpendicularto the yarn axis and fixed at that position for all twist varieties.

For a specific yarn 2 at particular adjustments relative to the blade 1and at a predetermined yarn speed there is a very high correlationbetween blade vibration frequencies and twist values of the yarn undertest. The regression coefficients (T/f) (where T=twist and f=frequencyof vibration) can easily be determined by conducting calibration testsrelated to the yarn under test and the speed to be used. If the testedyarn has regular twist, the blade vibration frequency will be constant.If the twist varies higher or lower than the nominal value then theresulting vibration frequency will be commensurately higher or lowerthan the frequency corresponding to the nominal twist.

Vibrations imparted to blade 1 are sensed by vibration sensing elements3 which output signals to converter 6 which transfers the vibrationfrequencies of blade 1 into twist values. Converter 6 includesappropriate electric circuits and filters providing the frequency ofvibration of blade 1 as sensed by elements 3, which may be strain gaugesor the like, due to the passage of yarn 2. Appropriate electric circuitsand filters provide the frequency of the signal which only correspondsto yarn twist and cut-off the frequencies due to the natural frequencyof the blade or due to the noise effects.

An embodiment constructed according to FIG. 1 using a steel blade38.0×8.0×0.25 mm. employs a pair of strain gauges 3 for sensing thevibrations of the blade, one is mounted on each side of the blade 1.Strain gauges 3 are coupled to a Wheatstone bridge included in a straingauge indicator; the output of the latter passes through a high-passfilter to cut off the frequencies due to noise and due to the electriccurrent, and through a low-pass filter to cut-off the natural frequencyof the blade. The output signal from the filters is received by auniversal counter to count the corresponding frequency within apreselected time interval and it displays the average frequency.According to this embodiment the natural frequency of the blade rangesto about 800 Hz. To avoid the interference of the natural frequency ofthe blade as well as of the noise, the band-pass filters have to be setup such that 200 Hz<working range<600 Hz. Therefore, this arrangementmay work successfully for plied or cabled yarns running at slow speeds.For example for cabled yarn/3 singles with nominal twist of 3 turns/cm,this embodiment may be used for continuous twist measurement for speedsbetween 13 and 40 m/min.

To overcome the speed limit and to apply this method for single yarns aswell, the device is modified using a very small (miniature) blade2.0×5.00×0.25 mm. For this embodiment it is more convenient to use piezoelectric bimorph as a vibration sensing element instead of the straingauges. FIG. 2 shows a schematic drawing of this embodiment wherein theyarn 2 passes across the edge of the miniature blade 1, which is gluedon one side of the piezo electric bimorph 3. The bimorph 3 is mountedcantilever fashion between the two poles 4 which are fixed in casing 5made of insulating material. The bimorph 3 and the poles 4 aresurrounded by rubber rings 7 as acoustic insulating packings. The piezoelectric bimorph 3 as a generator; it transfers the mechanical vibrationof the blade-end into alternating electric current at the poles 4. Forevery mechanical vibration of the blade 1, an electric wave of one cycleis produced at the pole 4. The output signal is received by theconverter 6 which includes appropriate electric circuits and filters toprocess the signal, display and/or compute the twist values. Using thisembodiment, the natural frequency of the blade could be higher than, say5 kHz. The working range of this version may be: 200 Hz<working range<4kHz which can accomodate twist measurements at higher speeds as well asthe twist measurements of single yarns. For example for the same cabledyarn mentioned before (3 singles with 3 turns/cm) this embodiment iscapable of measuring ply twist at speeds between 15 and 270 m/min.

Theoretical aspects of the present invention are explained in referenceto FIG. 2, where the yarn 2 is assumed to have a "helical multi-thread"on its surface with the following parameters:

m: number of helical threads on yarn surface=number of yarn surfaceelements, i.e. number of filaments or fibres on single yarn surface ornumber of single yarns on cabled or plied yarn surface.

T: yarn twist in turns/cm

h: lead of a helical thread, i.e. the length of one turn of twist in cm,or

    h=(l/T)                                                    (2)

p: pitch in cm, i.e. the distance between two consecutive crests, or

    T=(h/m)                                                    (3)

If the yarn 2 moves with a velocity of v cm/s crossing the blade 1,assuming every passing crest of the serrated profile of yarn surfacewill impart one pulse to the blade, then the blade will vibrate with afrequency f Hz, where

    f=(υ/p)                                            (4)

By substituting equations (2) and (3) in equation (4), then

    p=(f/mv)                                                   (5)

The value of (f/υ) may be considered as the measuring index I_(b) of theblade vibration device.

Therefore equation (5) could be written in the general form:

    T=K.sub.b I.sub.b                                          (6)

Where K_(b) is the proportionality constant.

Under ideal conditions and according to equation (5)

    K.sub.b =(l/m)                                             (7)

This means that in the ideal condition K_(b) takes the reciprocal valueof the number of yarn surface elements m. Its application is clear forply or cabled yarns where m equals the number of singles taking helicalpath on yarn surface. For yarns where the total number of constituentelements N (i.e. total number of singles in a cabled yarn) is notgreater than 5, then m=N, or

    K.sub.b =(l/N) for 2≦N≦5                     (8)

For cabled yarns with a total number of constituent singles greater than5 (or N>5) as well as for single yarns where the number of constituentfibres is mostly >5, in these cases m is not necessarily equal to N.This is because some of the yarn constituent elements exist inside theyarn body as a core and may not appear on the yarn surface. To determinem in these cases, the pattern taken by the yarn constituent elements hasto be found out. Accordingly m may be calculated.

Referring to FIG. 3 where there is shown apparatus of the type of FIG. 1or FIG. 2 mounted as an on-line twist sensing and monitoring componentof a control system in an open-end spinning process. The input fibrematerial 7, e.g. sliver, roving or tops, is fed to the twist insertiondevice 8, e.g., an open-end spinning rotor, and delivered as a spun yarnwith constant speed by the nip rollers 9. The yarn 2 runs with constantspeed over adjustable guide rollers 10 and 10a passing, in-between, overthe twist measuring device 12, in contact with the edge of sensing bladeedge 1. The yarn 2 passes through the nip rollers 9a to the winding head11 such that the yarn tension remains constant. The electric signalscoming out of the converter 13 (similar to the converter 6 in FIGS. 1 or2), which correspond the to the values of blade vibration frequenciesand consequently to the twist values, are integrated over apredetermined time period through integrator 14 and a signalcorresponding to the mean value is delivered to amplifier 15. The outputsignal can be received by a pen recorder 16 giving a chart representingthe continuous measurement of twist values and/or transmitted tocomperator 17. The rate of twist insertion is measured by coupling atachogenerator 20 to the motor 19, giving an electric signalcorresponding to the actual rotational speed of the spinning rotor 8.The comperator 17 is fed with three signals, namely an adjusted nominalvalue A, a measured value B and the actual twist insertion rate value C,and it delivers the resultant value D. The latter signal D istransmitted to the variable speed device 18 which in turn controls themotor speed so that the yarn twist produced is adjusted to the requirednominal twist value and maintained constant at this value.

It will be appreciated that the present invention is not limited by thespecifics of the preceding description in relation to the drawings andthe addressee will be aware of variations and modifications onceapprised of the essential elements of the invention.

It will be recognised by persons skilled in the art that numerousvariations and modifications may be made to the invention as describedabove without departing from the spirit or scope of the invention asbroadly described.

We claim:
 1. A method of measuring twist in a translating yarncomprising sensing the frequency of vibration of a fixed blade having anedge in contact with the translating yarn and calculating the twist ofthe yarn which is a function of the frequency of vibration of saidblade.
 2. A method as claimed in claim 1 wherein the edge of the bladein contact with the yarn is perpendicular to the axis of the yarn wherethe yarn contacts the blade.
 3. A method as claimed in claim 1 whereinthe edge of the blade in contact with the yarn is inclined to the axisof the yarn where the yarn contacts the blade, said inclination beingsuch that the amplitude of vibration of the blade, as the yarn istranslating, is at a maximum.
 4. Apparatus for measuring twist in atranslating yarn comprising a fixed blade adapted to have an edge incontact with a translating yarn, means for sensing the vibrationimparted to said blade by the translation of said yarn across said edge,and means for measuring the frequency of said vibration.
 5. Apparatus asclaimed in claim 4 wherein the means for sensing vibration of the bladecomprise strain gauges located on opposite sides of the blade. 6.Apparatus as claimed in claim 4 wherein the means for sensing vibrationof the blade comprises a piezo electric bimorph.
 7. Apparatus as claimedin claim 4 where the output of the sensing means is input to a converterwhich produces an output proportional to the twist of the translatingyarn.
 8. Apparatus as claimed in claim 7 where the output of theconverter is proportional to the instantaneous twist of the yarn. 9.Apparatus as claimed in claim 7 where the output of the converter isproportional to the average twist of the yarn for predetermined timeintervals or lengths of yarn as the yarn is travelling in contact withthe edge of the blade.
 10. An automatic control system for a twistinsertion apparatus comprising: a fixed blade adapted to have an edge incontact with a translating yarn; means for sensing the vibrationimparted to said blade by the translation of said yarn across said edge;means for measuring the frequency of said vibration; and, means forproviding a feedback signal controlling the twist insertion rate appliedto input fibre material at a twist insertion station.